1. Field of the Invention
The invention relates to an electromagnetic device for rectilinearly moving a nuclear reactor control bar.
2. Prior Art
Numerous electromagnetic devices developed for that use are already known. An electromagnetic device disclosed in French No. 1,371,802 and Belgian No. 753,529 for rectilinearly moving a shaft releasably secured to a nuclear reactor control bar comprises a fluid tight casing projecting from a cover of the reactor vessel. First and second sets of shaft gripping means, staggered in the longitudinal direction, are longitudinally movable along and within the casing. The first set of means is associated with a movable pole co-operating with a "holding" coil supported by the casing and is moved upon energization and de-energization of the coil between a position in which the first set of means grips the shaft and a position in which it releases it. The second set of means if associated with a rectilinearly movable plunger co-operating with a "transfer" coil and which moves upon energization and de-energization of this coil, between a position in abutment against another pole in which the set of means grips the shaft and a position in which it releases it, said other pole cooperating with a "lift" coil for moving said other pole between two positions spaced apart by a given pitch or step.
Prior art devices of the above-defined type operate satisfactorily. Upon energization of the different coils in a suitable sequence, the shaft and the control bar may be moved step-by-step in one direction or in the other. By supplying only the holding coil with power, the rod is held in a well-defined position. Should the power supply of all coils be cut off, the shaft is released and in the usual case where the bar is placed above the reactor, it falls up to the maximum insertion position in the core; consequently, a power failure affecting the drive devices of all bars causes an emergency stop of "scram".
It has been realised for a long time that devices of the above-described type, whose casing is completely filled with reactor coolant during operation, are a source of considerable heat losses. Convection and conduction phenomena cause heat transfer from the reactor to the casing. The casing must be cooled, in general by ventilation, for maintaining the coils at an acceptable temperature. In a 900 MWe reactor, which may be considered as representative, the total heat power dissipated by each bar drive device is about 8.5 kW. These losses require a powerfl blowing system for heat removal. They will be hardly acceptable in planned advanced reactors (undermoderated light water and/or spectral shift reactors for example) which will have an increased number of control bars.